Firefox 4+ (Windows only): Usually, changing your Firefox browser's looks requires a CSS tweak, an about:config switch, or specialized downloads. Stratiform is an all-in-one add-on that offers a variety of button, toolbar, and other visual element switches. Try out new themes and switch back without any hassle.

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As Stratiform's developer notes, these kind of tweaks required developer-level or deep-down changes before Firefox 4. With the latest release, add-ons like Stratiform make tweaking colors, dimensions, fonts, and other elements pretty simple. Click what you like, and the changes take effect instantly, with no harm to anything.

Stratiform is a free download for Firefox on Windows only, and requires Firefox 4 or later.